The study will examine the metabolism of exogenous fatty acids by Macaca fascicularis. Trans fatty acids which cannot be synthesized by eukaryotes will used as the exogenous fatty acid marker. These fatty acids are geometrical isomers of cis fatty acids which can be synthesized by eukaryotes. The trans fatty acids are catabolized and removed from the body shortly after the fatty acids are removed from the diet. Twelve ale M. fascicularis monkey were used in the study. Six were fed the control diet, and the others received the experimental diet. The RBC membrane of monkeys fed the control diet contained .5% trans octadecanoate isomers of the fatty acids. Examination of fecal fatty acids from these monkeys showed that biohydrogenation does occur in their gut producing trans fatty acids which appear to be incorporated into the RBC membrane. Within 6 weeks after initiating the study the RBC membranes of monkeys fed the experimental diet contained 10.8% trans octadecanoate isomers of the total fatty acids. DPII steady state fluorescence polarization analysis of the erythrocyte membrane fluidity showed no change in membrane fluidity between the monkeys fed the two diets. However, insulin binding activity was altered in the monkeys fed the diet containing trans fatty acids.